God bless AR!

order.rb

class Order < ActiveRecord::Base
 
  ## Setup accessible (or protected) attributes for your model
  attr_accessible :project_id, :developer_id, :due_date, :status, :notes, :is_delivered, :deliveted_at, :completed_at
 
  ## Virtual attributes
  attr_accessor :pages
 
  ## Associations
  belongs_to :project
  belongs_to :developer,  :inverse_of => :assigned_orders,
                          :class_name => "User"
 
  ## Callbacks
  before_save :assign_developer, :if => :developer_id_changed?
 
  ## Custom methods
  def assign_developer
 
    # we create an assignment for the new developer on the project
    # if it doesn't have one already
    self.project.assignments.create!(:user_id => self.developer_id) if self.project.assignments.find_by_user_id(self.developer_id).nil?
    
    # if the old developer doesn't have any other order assigned on this project
    # we should (?) delete the assignment to it
    self.project.assignments.find_by_user_id(self.developer_id_was).destroy unless self.project.orders.find_by_developer_id(self.developer_id_was).nil?
 
  end
 
end